In the art of deposition of films of material on a substrate, there are many known techniques, including vacuum deposition, ion plating, ion- and plasma-assisted, and the more modern Ionized Cluster Beam ("ICB") technique. ICB is an ion-assisted technique in which the material to be deposited on a substrate is heated in a closed crucible and its vapor ejected through a small nozzle into a vacuum region. The vapor forms loosely held clusters each comprising 100 to 2000 atoms. Some of the ejected vaporized material is ionized by electron bombardment and is accelerated toward the substrate, which is also disposed in the vacuum region. The material arrives at the substrate surface to be deposited thereon together with the neutral (non-ionized) component of the vapor. ICB offers the ability to control the deposited film structure by applying kinetic energy to the cluster vapor beam during film deposition. Kinetic energy control is achieved by varying the acceleration voltage and the electron current for ionization.
In most of the known ion- and plasma-assisted deposition techniques, the individual atoms of the material to be deposited on the substrate impact the substrate with too much kinetic energy, producing a high number of defects. However, with ICB a more useful lateral energy is obtained as the cluster impacts the substrate and the atoms break off. ICB deposition offers the possibility of getting useful energy into the film formation process without damaging the film and substrate surface. Due to the effects of ionized cluster bombardment, ICB produces films with high density, strong adhesion, a low impurity level, and a smooth surface. Further, some of the properties of the films usually associated with high substrate temperature in conventional vacuum depositions can be obtained at low substrate temperature in the ICB technique. This results in a distinct advantage in semiconductor device fabrication. U.S. Pat. Nos. 4,152,478 and 4,217,855 to Takagi describe and claim the ICB method and corresponding apparatus.
In the field of semiconductor processing, the aforementioned deposition techniques have been employed, all with varying disadvantages. Standard bulk silicon-based semiconductor technologies are inherently sensitive to elevated temperature operation as well as to the exposure of ionizing radiation to the large charge collection volumes within the devices. Also, the high parasitic capacitances in these devices tend to decrease operating performance (e.g., speed). These factors have prompted migration to a silicon-on-insulator ("SOI") substrate technology in which collection volumes and parasitic capacitances are dramatically reduced. Several SOI substrate technologies currently exist, e.g., SIMOX, ISE/ZMR, BESOI, FIPOS, etc. However, these technologies possess poor manufacturability or provide poor quality substrates. The result is an expensive substrate which is inadequate for most applications. For example, a five (5) inch SIMOX (i.e., "separation by implantation of oxygen") wafer costs approximately $500-800, compared to $40 for a five (5) inch bulk silicon wafer.
Semiconductor devices have been manufactured on silicon-on-sapphire ("SOS") substrates for several years. However, these SOS wafers are relatively expensive (approximately $500 each) and of very low quality. Additionally, these substrates are incompatible with bulk silicon processing tools due to their thickness, lack of flatness, poor silicon uniformity, and thermal sensitivity. The thickness of the sapphire substrate provides an extremely high hole trapping volume when exposed to total dose ionizing radiation. This complicates the radiation hardened ("rad-hard") aspects of the device design. Also, the sapphire substrates can only be made of a limited size (e.g., five (5) inch wafer diameter or less), making SOS incompatible with trends in VLSI processing.
In order to achieve optimum high temperature performance for semiconductors, a high quality but thin layer of device silicon is required. Up to now, currently available deposition techniques have not adequately provided the quality and thickness control required in these semiconductor applications.
It has been reported that aluminum oxide films of Al.sub.2 O.sub.3 (i.e., sapphire) have been prepared using ionized A1 clusters from an ICB source together with O.sub.2 introduced into a vacuum chamber. See Ito et al., "Ionized Cluster Beam Deposition Source for Aluminum and Aluminum Oxide Formation", Japanese Journal of Applied Physics, Vol. 30, No. 11B, Nov., 1991, pp. 3228-3232. The ICB apparatus described therein has no electron extractor. Such use of O.sub.2 is referred to as a reactive ionized cluster beam process, or R-ICB. A further example of the deposition of Al.sub.2 O.sub.3 using the R-ICB method is found in Sosnowski et al., "Ionized Cluster Beam Deposition and Thin Insulating Films", Nuclear Instruments and Methods in Physics Research, B46, (1990), pp. 397-404.
Sapphire, the highly stable oxide of aluminum, used in many applications due to its many advantageous features. Such features include high melting temperature, chemical stability and resistance to many commercial etchants, ease of maintaining proper stoichiometry, optical transparency, excellent dielectric properties, high thermal conductivity, ease of handling, relatively low cost, and relatively low deposition temperature. In the semiconductor industry, deposited films such as sapphire, crystalline polysilicon ("poly"), or aluminum alloys are used to create many of the wiring and insulating layers. After the metal layers are patterned, dielectric films are used to electrically isolate one metal layer of wiring from another. In this context, the dielectric properties of the film are highly important. Also, the ability to conduct heat away from the active devices or from the metal wiring is significant from a reliability standpoint.
However, in both of the above reported Al.sub.2 O.sub.3 ICB deposition techniques, the starting material was aluminum and not sapphire. The aluminum was vaporized and accelerated through the oxygen environment. It is well known that aluminum in an oxygen ambient is highly reactive. Thus, the crucible containing the aluminum starting material typically comprises a more costly tungsten material instead of a lesser expensive graphite. Also, the oxygen introduces yet another complexity, i.e., that of uniformly controlling the flow rate of oxygen across the diameter of the wafer. Any resulting perturbations in the flow rate can cause a lack of uniformity in the deposition of the resulting Al.sub.2 O.sub.3 on the substrate. Therefore, up until now, there has not been reported any deposition of Al.sub.2 O.sub.3 directly onto a silicon substrate using the ICB methodology.